Add differenceBy

snippets/differenceBy.md

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+### differenceBy
+
+Returns the difference between two arrays, after applying the provided function to each array element of both.
+
+Create a `Set` by applying `fn` to each element in `b`, then use `Array.filter()` in combination with `fn` on `a` to only keep values not contained in the previously created set.
+
+```js
+const differenceBy = (a, b, fn) => {
+  const s = new Set(b.map(v => fn(v)));
+  return a.filter(x => !s.has(fn(x)));
+};
+```
+
+```js
+differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1.2]
+differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [ { x: 2 } ]
+```